fix lint (PSScriptAnalyzer) issues (#62)

#### 📖 Summary

<!-- Provide a summary of your changes. Describe the why and not how. -->

#### 📑 Test Plan

> 💡 Select your test plan for the code changes.

- [x] Tested via Drone.io pipeline
- [ ] Custom test
- [ ] No test plan

##### Details / Justification

<!-- Add your test details or justification for missing tests here. -->

#### 📚 Additional Notes

<!-- A place for additional detail notes. -->

Co-authored-by: OCram85 <marco.blessing@googlemail.com>
Reviewed-on: OCram85/PSCredentialStore#62

src/Private/Test-CSPfxCertificate.ps1

@@ -64,6 +64,8 @@ function Test-CSPfxCertificate {
     }
 
     process {
+        # Script analyzer issue (unused var) workaround
+        $null = $Thumbprint
         $Cert = $Store.Certificates | Where-Object { $_.Thumbprint -eq $Thumbprint }
 
         if ($null -eq $Cert) {